This invention is directed to electronic conversion systems and, more particularly, to current (or voltage)-to-frequency converters.
In many electronic circuits and systems, precise current (or voltage)-to-frequency conversion is necessary. For example, in an inertial navigation system, the position of the vehicle is determined by measuring vehicle accelerations and angular rates from a starting point. Usually, accelerometers and gyroscopes are used to sense vehicle acceleration and angular rates. These devices produce electric current outputs whose magnitudes are (nearly) proportional to their respective input variables. In order for such electrical currents to be usable by a suitable information manipulating system, such as a digital computer, they must be first placed in a form suitable for use by such a system. One way to convert electrical current information into such a form is to convert the current information into frequency information. For example, the current information may be converted into a pulse train having a frequency rate proportional to the magnitude of the current at any particular point in time. The present invention is directed to a current-to-frequency (I/F) converter suitable for performing this function.
While the invention was developed for use in combination with precision accelerometers and gyroscopes used in inertial navigation systems, it will be readily appreciated by those skilled in the art and others that the invention is also suitable for use in other environments. Thus, the herein described invention should not be construed as limited to use in such environments. In addition, while the invention is described as adapted to convert current information into frequency information, it will readily be appreciated that current and voltage are interchangeable parameters when appropriate resistive elements are used. Thus, the invention is also useful as a voltage-to-frequency converter. As such, the term current, as used herein, is generic, i.e., it covers both current and voltage-to-frequency converters.
There are a number of constraints placed upon I/F converters useful in inertial navigation systems, and other similar environments. For example, when used in a strapped down inertial navigation system, an I/F converter must have an exceptionally wide dynamic range (typically 10.sup.8). In addition, it must have low bias drift, good linearity and good scale factor stability. Moreover, the asymmetry of the converter must be low, i.e., the difference in output frequency for input currents of opposite direction but equal in magnitude must be small (typically in the order of .+-.5 parts per million).
Prior to the present invention, these strict requirements have been met by using high stability components (resistors, amplifiers, switches, etc.) and by temperatures stabilizing the environment surrounding the converter. With respect to temperature stabilization, often the converter has been placed, either partially or entirely, inside of a temperature controlled oven. Even with these precautions, however, it has been difficult to meet the strict requirements noted above, particularly the requirement for low asymmetry.
Therefore, it is an object of this invention to provide a new and improved bi-directional converter.
It is also an object of this invention to provide a current (or voltage)-to-frequency converter that is suitable for use in environments wherein inputs currents may flow in either direction.
It is another object of this invention to provide a precision bi-directional converter having low asymmetry, good linearity, low bias drift, a wide dynamic range and good scale factor stability.
Reset integrator type current-to-frequency converters have been known for a number of years. Conversion is accomplished by letting the current to be converted charge a capacitor connected in the feedback loop of an integrating amplifier. When the output of the amplifier reaches a predetermined level the capacitor is partly discharged by injecting a charge of opposite polarity into the capacitor. The charge may be generated by using either the relationship Q = V .sup.. C or the relationship Q =to I .sup.. .DELTA.T: where V = voltage, C = capacitance, I = current and .DELTA.T = time interval.
The first relationship noted above is normally carried out in a practical structure by providing a low impedance path connected in parallel with the charged capacitor and shorting the capacitor for timed intervals. More specifically, each time the capacitor is charged to some predetermined level, it is discharged for a predetermined time period by connecting it to the low impedance path. The problem with this approach is that the scale factor stability of the overall converter is dependent upon the stability of the capacitor and the reset voltage level. Not only is the scale factor unstable, it is also non-linear because the low impedance path across the capacitor also has the effect of short circuiting the input current during the reset (shorting) intervals.
The second relationship noted above avoids the problem of capacitor stability, and good linearity is obtained. However, systems utilizing this relationship in the past have not been satisfactory when the converter is to be used to convert bi-directional currents because two physically different circuits have been utilized to provide the Q =I .sup.. .DELTA.T charges, one for each current direction. In this regard, attention is directed to U.S. Pat. No. 3,040,273 issued June 19, 1962 to A. F. Boff, for Voltage To Frequency Converter. The primary disadvantage of such systems is that it is virtually impossible for them to achieve good asymmetry performance over a wide temperature range. In the past satisfactory, but not good, asymmetry has been obtained at the expense of placing such converters in a temperature controlled environment.
Therefore, it is yet another object of this invention to provide a current (or voltage)-to-frequency converter for converting currents of either direction that has good asymmetry over a wide temperature range.
It is still another object of this invention to provide a precision current-to-frequency converter that has good linearity, low asymmetry, good scale factor stability with temperature and operates over a wide dynamic range with low bias drift regardless of the direction of the current to be converted.